1. Field of the Invention
The present invention relates generally to optical information reading apparatus for reading from an optical disk such as a compact disk or a laser disk with information recorded thereon, and more particularly, to an optical information reading apparatus employing the wedge prism method in a focusing servo.
2. Description of the Background Art
On the surface of an optical disk such as a compact disk and a laser disk, small hollows referred to as pits are spirally arranged from the central portion of the disk to the peripheral portion thereof. All pits have the same width. However, the length of each of the pits and an interval between pits respectively differ depending on the recorded information. The surface of the disk is covered with a reflecting film formed by, for example, evaporation of aluminum. When the surface of this disk is irradiated by light, light incident on the pits is absorbed while light incident on a surface having no pits is reflected. In an optical information reading apparatus utilizing such a principle, a laser beam is first converged by a lens so that light spots are formed and the, the formed light spots are irradiated along a pit train on the surface of the optical disk while rotating the optical disk, whereby the presence or absence of pits is detected by detecting the presence or absence of reflected light. More specifically, information recorded on the optical disk can be read.
FIG. 1 is a schematic diagram showing a structure of a conventional optical information reading apparatus using a holographic grating. This apparatus employs the wedge prism method in a focusing servo for irradiating light spots having the same shape which are always in the in-focus state on an optical disk.
Prior to the description of FIG. 1, the principle of the wedge prism method will be briefly described. The wedge prism method basically employs a wedge prism 200 assuming the shape of a V-shaped valley in a cross section as shown in FIG. 2A. Referring to FIG. 2B, a beam of light is divided into two sections in the central portion of the wedge prism 200. Each of the beams of light divided by the prism 200 forms a dot-shaped light spot in an in-focus position F while forming a semicircular spot in positions X and Y before and after the in-focus position F. Referring to FIG. 2B, the light spot formed from each of the divided beams (of light) assumes a semicircular shape expanded outward in the position X before the in-focus position F while assuming a semicircular shape expanded inward in the position Y after the in-focus position F. According to the wedge prism method, such a change in shape of the light spot caused by offset from the in-focus position is detected as a change in electrical signal so that a focus error signal is obtained.
In the apparatus shown in FIG. 1, a holographic grating 2 is used instead of the wedge prism. FIG. 3 is an enlarged view of the holographic grating 2.
Referring to FIG., 3, the holographic grating 2 has curved grooves which are diffraction gratings on a glass or plastic plate member so as to branch a part of a beam of incident light. The holographic grating 2 is divided into two regions 2a and 2b by a dividing line 7. Regions of and pitches between the grooves respectively formed int he two regions 2a and 2b respectively differ from each other. Therefore, the angles of diffraction of beams of light respectively incident on the regions 2a and 2b differ from each other, so that first-order diffracted beams are converged in different positions. More specifically, since two diffraction gratings having different shapes are formed in the holographic grating 2, the holographic grating 2 has a function of branching the beam of light into two, i.e., the same function as that of the wedge prism.
Referring to FIG. 1, the optical information reading apparatus comprises an optical disk 4, an optical system 12 for reading information on the disk 4, an RF detecting portion 11 for detecting from an output of the optical system 12 an RF (Radio Frequency) signal representing information recorded on the disk 4, and a servo system 13 for controlling the optical system 12 in response to the output of the optical system 12. The optical system 12 comprises a source of laser light 1, a holographic grating 2 which is a diffraction grating element, a collimating lens 3a for turning incident light into parallel light, an objective lens 3b for forming light spots by a laser beam on the disk 4, and a photosensitive detector 8 for receiving diffracted light from the holographic grating 2 to convert the same into an electrical signal.
In FIG. 1, a laser beam emitted from the source of laser 1 is incident on the holographic grating 2. A zero-order diffracted beam out of beams of light incident on the holographic grating 2 is directed to the collimating lens 3a. In this case, there also exists a first-order diffracted beam. However, the angle of diffraction thereof is large so that the first-order diffracted beam does not reach the collimating lens 3a. The collimating lens 3a turns the incident zero-order diffracted beam into parallel light and directs the same to the objective lens 3b. The light incident on the objective lens 3b is converged, to form light spots 5 on the surface of the optical disk 4. A pit train 6 representing recorded information is formed on the surface of the optical disk 4 in a circumferential direction of the optical disk 4. The light (light spot 5) incident on the optical disk 4 is absorbed on the surface of the optical disk 4 when the light spots 5 are formed on pits 61 (constituting the pit train 6) while being reflected when the light spots 5 are not formed on the pits 61. This reflected light is incident on the holographic grating 2 again through the same optical path. On this occasion, the holographic grating 2 comprises two regions 2a and 2b divided by a dividing line 7 (although the dividing line exists in the drawing to show the boundary, no such line actually exists) parallel to an tangential direction of the optical disk 4. Thus, first-order diffracted beams of light incident on the regions 2a and 2 b respective form light spots 9a and 9b on the photosensitive detector 8. More specifically, the first-order diffracted beam of light incident on the region 2a of the holographic grating 2 out of the light reflected from the optical disk 4 is converged to form the light spot 9a. Similarly, the first-order diffracted beam of light incident on the region 2b forms the light spot 9b on a light receiving surface of the photosensitive detector 8. The zero-order diffracted beam from the holographic grating 2 is returned to the source of laser. The photosensitive detector 8 outputs electrical signals proportional to the intensity of light of the light spots 9a and 9b. Thus, if an output of the photosensitive detector 8 is detected, the presence or absence of pits on the optical disk 4 can be detected.
Description is now made of a structure of the photosensitive detector 8. FIG. 4 is an enlarged view of the photosensitive detector 8. Referring to FIG. 4, the photosensitive detector 8 comprises photosensitive detectors 81 to 84 divided by dividing lines 14a, 14b and 14c parallel to a tangential direction of the optical disk 4. Thus, if outputs of the photosensitive detectors 81 to 84 are added, an RF signal corresponding to information recorded on the optical disk 4 is detected.
Turning to FIG. 1, the RF signal detecting portion 11 electrically adds output signals of the photosensitive detectors 81 to 84 to provide an RF signal.
Then, the servo system 13 in this optical information reading apparatus comprises an FES detecting portion 10a for outputting a focus error signal (referred to as FES hereinafter) in response to an output of the optical system 12 and a focusing adjustment driving portion 10b for driving a focusing adjusting mechanism in response to an output of the FED detecting portion 10a. In this apparatus, the above described wedge prism method is employed. Thus, light spots having different shapes depending on the in-focus state or the out-of-focus state are formed on the photosensitive detector 8, as described above. FIG. 5A illustrates a state in which images of light spots 9a and 9b are formed in a photosensitive detector 8 when the distance between the optical system 12 and the optical disk 4 is a predetermined distance, i.e., if the distance between the objective lens 3b and the optical disk 4 is a predetermined distance (in the in-focus state). In the in-focus state, images of first-order diffracted beams from the two regions 2a and 2b of the holographic grating 2 are respectively formed in the form of spots 9a and 9b between photosensitive detectors 83 and 84 and between photosensitive detectors 81 and 82 in the photosensitive detector 8. If the distance between the optical system 12 and the optical disk 4 is shorter than a predetermined distance, the light spots 9a and 9b formed on a light receiving surface of the photosensitive detector 8 have shapes as shown in FIG. 5B. The positions of the images are formed offset to the side of the photosensitive detectors 84 with respect to a dividing line 14a for dividing the photosensitive detectors 83 and 84 and to the side of the photosensitive detector 81 with respect to a dividing line 14b for dividing the photosensitive detectors 81 and 82. FIG. 5C illustrates a state in which images of light sports 9a and 9b are formed when the distance between the optical system 12 and the optical disk 4 is longer than a predetermined distance. The light spots 9a and 9b are respectively formed in positions offset to the side of photosensitive detectors 83 and 82 with respect to dividing lines 11a and 11b. The formed light spots respectively assume inverted semicircular shapes expanded to the side of the photosensitive detectors 83 and 82.
Thus, the FES is obtained by detecting the difference between the sum of outputs of the photosensitive detectors 81 and 84 and the sum of outputs of photosensitive detector 82 and 83.
FIG. 6 is a diagram showing one example of a method of forming an FES in the FES detecting portion 10a shown in FIG. 1. Referring to FIGS. 1 and 6, the FES detecting portion 10a comprises a differential amplifier 100 for amplifying the difference between the sum of outputs from both photosensitive detectors 81 and 94 and the sum of outputs from both photosensitive detectors 82 and 83 to output the same as an FES. The focusing adjustment driving portion 10b displaces the distance between the optical system 12 and the optical disk 4 in response to the FES of the FES detecting portion 10a. Thus, if the difference between the sum of the outputs of the photosensitive detectors 81 and 84 and the sum of the outputs of the photosensitive detectors 82 and 83 is zero, i.e., in the in-focus state, the FES is not outputted so that the focusing adjustment driving portion 10b does not displace the optical system 12.
In the above described conventional optical information reading apparatus, the objective lens 3b is made of materials such as glass and plastic. However, it is preferable that the objective lens 3b is made of plastic material in terms of ease of manufacture, cost and quantity production. However, when the objective lens 3b is made of plastic, astigmatism is developed in the objective lens 3b depending on errors and the change in temperature in forming the objective lens 3b. In such a case, light spots 5 formed on the optical disk 4 are elliptical in shape. In this case, a state in which the objective lens 3 is arranged is adjusted such that the direction of the long axis of the elliptical light spots 5 is at right angles to a track provided with a pit train 6, i.e., a circumferential direction of the optical disk 4, thereby to make steep the change in output of the photosensitive detector 8 depending on the presence or absence of pits. FIG. 7A is a diagram showing the positional relation between pits 61 and a spot 5 in the above described case. FIG. 7B is a diagram showing a state in which images of spots 9a and 9b are formed when the distance between the optical system 12 and the optical disk 4 becomes a predetermined distance, i.e., in the in-focus state, if an elliptical spot 5 is formed on the surface of the optical disk 4. Since light reflected from the optical disk 4 to be incident on a holographic grating becomes a beam of light assuming an elliptical shape in cross section, a light spot 9a offset to the side of a photosensitive detector 84 (or offset on the side of a photosensitive detector 83) with respect to a dividing line 14a and a light spot 9b offset on the side of the photosensitive detector 84 (or offset on the side of a photosensitive detector 82) with respect to a dividing line 11b are formed on a light receiving surface of a photosensitive detector 8. In such a state, the difference between the sum of outputs of the photosensitive detectors 81 and 84 and the sum of outputs of the photosensitive detectors 82 and 83 is not zero, so that a focus error signal is provided irrespective of the in-focus state. Consequently, the focusing adjustment driving portion 10b displaces the optical system 12 so as to move nearer to or move further away from the optical disk 4. Therefore, the distance between the object lens 3b and the optical disk 4 becomes shorter or longer than a predetermined distance in the in-focus state. Consequently, no good spots are formed on the optical disk 4 so that no goof RF signal can be obtained. As a result, information recorded on the optical disk 4 cannot be precisely read.
In order to avoid erroneous output of the focus error signal as described above, it is necessary to make a positional adjustment of the objective lens 3b and the holographic grating 2 in combination such that the light sports 9a and 9b formed of first-order diffracted beams from the holographic grating 2 are respectively located between the photosensitive detectors 83 and 84 and between the photosensitive detectors 81 and 82 in the in-focus state when the objective lens 3 having an astigmatism is used. However, in order to possibly make such a positional adjustment, the structure of the optical system must be complicated. As a result, when the above described positional adjustment is made, the structure of the entire apparatus is complicated. In addition, since the astigmatism of the objective lens 3b is changed depending on the change in temperature, the shapes of the light spots formed on the disk 4 are not constant. Thus, the above described positional adjustment is not necessarily made effectively, so that it is difficult to improve the reliability of the apparatus.